Musical Fidelity A1 (2008) power amp clone project

[panson_hk]

The reverse engineered power amp circuit of A1 2008 edition is published in a Chinese magazine. The output devices are STD03P/N. I simulated in LTspice with discrete Darlington. The performance is very promising. The amp uses two-pole compensation to achieve good THD performance. I am going to build a test board.

The simulation circuit is shown here. R25 and 26 are not present in the actual circuit.

[Sebastiaan]

Originally Posted by panson_hk

The reverse engineered power amp circuit of A1 2008 edition is published in a Chinese magazine. The output devices are STD03P/N. I simulated in LTspice with discrete Darlington. The performance is very promising. The amp uses two-pole compensation to achieve good THD performance. I am going to build a test board.

The simulation circuit is shown here. R25 and 26 are not present in the actual circuit.

Very interesting Panson and thanks for sharing with us. It is no a secret that I love the sound of the STD03's Do you know why they use additional D4 and D5? Can't the diodes inside the STD's provide sufficient voltage drop for high bias? (I never biased the STD03's so high so I have no clue).

I will follow you close with this project, since I can learn a lot from your experiences. Especially the biasing, because I failed to let the STD03's work well with high bias levels. I look forward to see how you going to make it work , but I am confident you will succeed.

Good luck!

With kind regards,
Bas

[panson_hk]

Originally Posted by Sebastiaan

Do you know why they use additional D4 and D5? Can't the diodes inside the STD's provide sufficient voltage drop for high bias? (I never biased the STD03's so high so I have no clue).

Hi Bas,

D4 and D5 are used for biasing the output in Class A. I will be very careful with STD in high bias.

Panson

[abraxalito]

Originally Posted by panson_hk

The simulation circuit is shown here. R25 and 26 are not present in the actual circuit.

Curious about those two input transistors. Is there a special reason to use ZTX757s and not to degenerate them with R25 and R26? As far as I could recall from Doug Self's book, the only downside for degenerating the input LTP was a noise penalty. But you can't be aiming for ultra-low noise here as your feedback components are fairly high impedance and you're running only 700uA into the tail. Are you intending to run at much higher voltages? - that would explain choosing 300V transistors for the input stage.

[Sebastiaan]

Originally Posted by panson_hk

Hi Bas,

D4 and D5 are used for biasing the output in Class A. I will be very careful with STD in high bias.

Panson

Forgive me for being so stubborn. I have no doubt Musical Fidelity knows what they are doing, I just want to understand "how" that is why I ask so much. I just don't understand how they keep the thermal tracking stable, because it is clearly stated and showed in the datasheet of the SAP devices that thermal tracking isn't good anymore above 80mA. I would be more then happy if the solution comes out here on the forum, so I can finally bias my own STD03's higher as well Measurements I did showed, that the THD get significant lower with high bias level. I was just never able to get it thermal stable by only rely on the thermal tracking diodes.

With kind regards,
Bas

[panson_hk]

Originally Posted by abraxalito

Curious about those two input transistors. Is there a special reason to use ZTX757s and not to degenerate them with R25 and R26? As far as I could recall from Doug Self's book, the only downside for degenerating the input LTP was a noise penalty. But you can't be aiming for ultra-low noise here as your feedback components are fairly high impedance and you're running only 700uA into the tail. Are you intending to run at much higher voltages? - that would explain choosing 300V transistors for the input stage.

I actually got the same question. However, this is the circuit shown in the article. I did not make any change except output Darlington by discrete. It may be due to the author did not make the reverse engineering correctly. However, there are many commercial amps not following D. Self's book.

Well, I will compare the performance between with and without degeneration when I build a prototype (and simulation).

[abraxalito]

Originally Posted by panson_hk

I actually got the same question. However, this is the circuit shown in the article.

Cool . I'd be inclined to make some improvements myself if laying out a PCB. Definitely include spaces for those resistors - they'll improve the slew rate and may even help in RF rejection (my favourite hobby horse). I feel certain that other Zetex (Diodes) transistors will do the job better than those 757s - their hFE below 1mA IC could be below 40.

[panson_hk]

Originally Posted by Sebastiaan

Forgive me for being so stubborn. I have no doubt Musical Fidelity knows what they are doing, I just want to understand "how" that is why I ask so much. I just don't understand how they keep the thermal tracking stable, because it is clearly stated and showed in the datasheet of the SAP devices that thermal tracking isn't good anymore above 80mA. I would be more then happy if the solution comes out here on the forum, so I can finally bias my own STD03's higher as well Measurements I did showed, that the THD get significant lower with high bias level. I was just never able to get it thermal stable by only rely on the thermal tracking diodes.

With kind regards,
Bas

Hi Bas,

Do you think that the STD used as conventional Darlington (omitting diode stack) will have problem for high bias? If the problem is solely due to diode stack temp coeff difference from that of the transistor, we should be able to find a proper diode stack current for good thermal tracking.

On the other hand, is Darlington not good for Class A operation since the driver is "over" heated by the output transistor. Have you seen/tried other Darlington for Class A operation?

Panson

[Sebastiaan]

Originally Posted by panson_hk

Hi Bas,

Do you think that the STD used as conventional Darlington (omitting diode stack) will have problem for high bias? If the problem is solely due to diode stack temp coeff difference from that of the transistor, we should be able to find a proper diode stack current for good thermal tracking.

On the other hand, is Darlington not good for Class A operation since the driver is "over" heated by the output transistor. Have you seen/tried other Darlington for Class A operation?

Panson

Dear Panson,

I've never seen a Darlington for class A operation till you showed me the A1 2008 edition schematics. I was aware though that Musical Fidelity builds even bigger class A amps with the STD03 device.

In a very optimistic attempt to make my own STD03's class A I failed the hard way, and blow them up many times. Not because of oscillation, but because of thermal run away. The amp goes into a thermal loop and heat up very rapid.

But, assuming the A1 2008 edition works good, it seems it must be possible with the STD03's. I just don't have the skills to make it happen in class A with those devices. I hope you provide the solution

EDIT: Maybe I overlooked it, but a bit tricky is indeed we don't know the hFE and the current ability of the build in drivers. Makes it indeed more tricky. But then again Musical Fidelity did it...

With kind regards,
Bas

[a.wayne]

Originally Posted by panson_hk

Hi Bas,

D4 and D5 are used for biasing the output in Class A. I will be very careful with STD in high bias.

Panson

High Bias has STD ? ..........................